The present invention relates to a coating film forming apparatus, which applies a paint to an outer circumferential face of a cylindrical-shaped object to be coated to form a coating film. Particularly, the invention relates to a coating film forming apparatus, which is suitable for forming an elastic layer for fixing members such as a fixing roller and a fixing belt to fix an unfixed toner image on a transfer paper by heating and pressing, for example, in image forming apparatuses, such as a PPC (Plain Paper Copier), an LBP (Laser Beam Printer), a facsimile and so on, in which an electrophotographic system is adopted.
In image forming apparatuses such as a copier, a printer, etc., based on a principle of electrophotography, a transfer paper where a toner image has been transferred, is passed between a heated fixing member and a pressurizing member which press-contacts with the fixing member, to melt toner of the toner image and to fix the toner image on the transfer paper, so as to perform a fixing process. The fixing member, which is used in the fixing process such as a fixing roller, a fixing belt or the like, is obtained by applying a primer (an adhesive), to a cylindrical-shaped core metal, comprising aluminum, iron and so on, or to an endless-shaped base body comprising polyimide, nickel and so on, and then by applying a paint having a heat-resistant rubber such as a silicone rubber and the like on the primer, and thereby an elastic layer having a thickness of about 100-300 μm is formed. (For example, refer to Japanese patent publication No. 2002-14557.)
The elastic layer is generally known to equalize a pressure to press toner on a transfer paper and improve a degree of granularity of an image, in the fixing process described above. Additionally, a thickness of the elastic layer is required to be even, because the thickness of the elastic layer influences a fixed image quality and a start-up time of the fixing member (a time which takes to reach a predetermined temperature).
As a method of forming the elastic layer described above, a dipcoating, a spray coating, a blade coating, a roll coating and so on are known.
The dipcoating method is relatively simple in that an object to be coated such as the cylindrical-shaped core metal, the endless-shaped base body or the like is dipped into a liquid tank filled with a paint, and after that, the coated object is moved up and down, and thereby the paint is applied to the coated object. However, in the dip coating method, there are some problems as follows. For example, when dipping, a difference of a layer thickness between a top part and a bottom part of the coated object occurs. In addition, when a paint with a high viscosity is applied to the coated object as a thin film, production efficiency is low, because a viscosity of the paint determines a speed of up-and-down movement (an applying speed) of the coated object. However, even when the paint with the high viscosity is diluted with a solvent and the like to make the viscosity lower, an unevenness of the thickness of the paint occurs in the liquid tank, and it is very difficult to maintain a liquid property.
The spray coating method described above is an applying method of spraying a paint in the form of a mist. However, there are some problems as follows. For example, when the paint with a high viscosity is used, it is required to be diluted with a solvent and the like. In addition, a smoothness of a coating film is inferior, because of blowing a sprayed paint onto the coated object. Furthermore, an arrival rate of the paint to the coated object is about several percent to 30%, and thus a yield of the paint is extremely low and the production efficiency is low.
The blade coating method and the roll coating method described above are applying methods in which a blade or a roller is arranged in an axial core direction of the coated object, the paint is applied to the coated object while the coated object is rotated, and the blade or the roller is moved back after one rotation of the coated object. However, there are some problems as follows. For example, when the blade or the roller is moved back, a part of a coating film becomes thicker than other parts of the coating film because of the viscosity of the paint, and thereby an even coating film cannot be obtained. Additionally, this partial uneven coating film may be a direct cause of triggering a low quality of a fixed image such as a partial lack of the fixing, a partial unevenness of light and shade and so on, especially in the electrophotographic fixing member.
The applying methods described above are not always suitable for a production of the elastic layer on the fixing member, in which the paint such as a silicone rubber with a high viscosity and the like is required to be evenly applied as a thin film, and are not efficient for mass productivity.
To solve the problems described above, an annular applying method, for example, in which a paint is discharged from an annular nozzle along an outer circumferential face of an object to be coated, and the paint is applied by relatively moving the nozzle and the coated object, has been disclosed (See, for example Japanese patent examined publication No. H07-52296 and Japanese patent publication numbers 2005-152830 and S56-15866).
Japanese patent examined publication No. H07-52296 and Japanese patent publication numbers 2005-152830 and S56-15866 disclose a paint applying method, in which an annular applying nozzle having a slit, which discharges a paint throughout a whole circumference of the coated object, where an axial core thereof is supported parallel to a vertical direction, is provided, and the coated object is relatively moved up in the vertical direction to the annular applying nozzle while discharging the paint, and thereby the annular applying nozzle applies the paint to the outer circumferential face of the coated object. In the applying method using this annular applying nozzle, the paint is not exposed to air just before the paint discharges from the annular applying nozzle, and which makes it easily possible to accurately manage a viscosity, a surface tension, a density and a temperature of this paint, and makes it possible to maintain a smoothness and an evenness of a coating film well. Additionally, it is possible to increase the applying speed by adjusting a supply of the paint, and productivity is significantly better compared to the dipcoating method described above. Furthermore, most of the discharged paint reaches or adheres to the outer circumferential face of the coated object, so that the yield of the paint is high.
However, in applying methods disclosed in Japanese patent examined publication No. H07-52296 and Japanese patent publication numbers 2005-152830 and S56-15866 described above, there is a problem in that the paint does not reach or adhere to the outer circumferential face of the coated object, when an interval between the coated object and the annular applying nozzle becomes larger than a certain interval. Therefore, it is necessary to install plural coating film forming apparatuses, in which a distance between the annular applying nozzle and the outer circumferential face of the coated object is adjusted according to the diameters of each object, when a coating film is to be formed on various types of the coated object having different diameters, and there is a tendency to increase the cost of equipment.
In addition, the coated objects are not only formed in a cylindrical shape having the same diameters all along the axial core direction from one end to the other end, but also are formed in an hourglass shape (inverted-crown shape) having a smaller diameter at the center in the axial core direction than at the end in the axial core direction, and moreover, are formed in a crown shape-having a smaller diameter at the end in the axial core direction than at the center in the axial direction. However, in the applying methods disclosed in Japanese patent examined publication No. H07-52296 and Japanese patent publication numbers 2005-152830 and S56-15866 described above, there are problems in that, for example, the paint does not reach the center of the hourglass-shaped coated object and a thickness becomes thinner at the center of the crown-shaped coated object than at both ends of the crown-shaped object. Therefore, it is difficult to apply the paint to the coated objects, in which diameters vary along the axial core direction.